1. Field of the Invention
The present invention relates to managing device information for digital memory devices, and more particularly to methods and apparatus for providing accessible user device information in digital memory devices.
2. Description of Related Art
Various types of digital memory devices have sectors that are formatted with user information useful for interfacing with the device. For example, the device information format provided in each sector of the NexFLASH NXS series serial memories available from Nexcom Technology, Inc. of Sunnyvale, Calif., includes sector tag/sync bytes, a device optimization code pattern, a device identification field, and a restricted sector address list. In the serial memory product number NX26F080, for example, each of up to 2048 sectors are pre-programmed during manufacture with the same device information format. While the device information format is therefore available in every sector, the manufacturer recommends that the last two sectors be maintained by the user as read-only locations for use as device information sectors for such purposes as assuring that the optimization codes, device identification, and restricted sector addresses are available for in-system use. Detailed descriptions of the device information format and of how to work with sector tag/sync bytes, device optimization code patterns, device identification fields, and restricted sector address lists are provided in two documents by Nexcom Technology, Inc., the NX26F080 Data Sheet Version 1.6, March 1996, and the application note entitled Developing With the NX26F080, AN-1, April 1996, which are incorporated herein by reference in their entirety. However, as the device user is responsible for ensuring that the device information sectors are not overwritten during use in the application, the device information sectors were sometimes inadvertently lost.
Another approach to storing user memory device information is described in U.S. Pat. No. 4,451,903, issued May 29, 1984 to Jordan and entitled "Method and Device for Encoding Product and Programming Information in Semiconductors." The device described in the Jordan patent uses multiple voltage levels on one of the row access pins to carry out the dual functions of accessing a memory array and an information array at appropriate times. The information array stores product information in read-only memory ("ROM") cells preferably of the diffusion masked ROM type, which cannot be altered once the circuit has been manufactured. Although the use of EPROM and E2PROM cells in the information array is contemplated, ROM cells are preferred since the date stored in EPROM and E2PROM cells would be entered after fabrication of the chip and thus subject to the entry of human error. The ROM cells are juxtaposed on a semiconductor die with the circuitry which performs the primary function of the chip, which includes the memory array and associated decoders, gates and buffers. Information such as manufacturer, mask set and other manufacturer-related information, as well as key circuit parameters such as supply voltages, operating currents, programming voltages, programming pulse-widths, and the like is stored in the ROM cells. To ensure that access to the data stored in the information array does not interfere with the normal operation of the primary circuit on the chip, array access logic is provided as an interface between the row address pin and the row decoder and information array. The access logic determines whether a high voltage signal is present, and disables the information array to prevent interference and enables the row decoders if conventional levels are present, and conversely disables the row decoders to prevent interference and accesses the information array if a high voltage outside of the normal voltage range for the device is present.
While ROM cells are useful for storing information predetermined by design, they are not useful for storing user device information that cannot be obtained until the device has completed testing and preparation for shipment. An example of such information is the restricted address list, which is stored in the device information sector as shown in the previously mentioned NX26F080 documents. Restricted sector devices have a limited number of sectors that do not meet manufacturing programming criteria over the specified operating range. For such devices, a restricted sector list provides a means of identifying sectors in the main array that have been determined during the manufacturer's testing to be defective and therefore appropriately restricted from use. One way of using the restricted sector list is to write the first byte of a non-restricted sector with a Tag/Sync value to identify a fully functional sector. Restricted sectors that are listed in the restricted sector list would have their first byte written to a value other than the Tag/Sync value. This easily identifies the sector as restricted or not restricted, as the case may be. It will be appreciated that the method of working with the Sector Tag/Sync bytes to assemble the restricted sector address list cannot be performed prior to mask ROM programming and completion of device fabrication. This also applies for other test-related information such as voltage, speed, and temperature grade, as well as customer and serial number identification.
Unfortunately, providing user device information in sectors of main memory places the vital device information in danger of being overwritten and corrupted. While using a high voltage outside of the normal device operating range on a multiple purpose pin to access device information in the application avoids danger of corruption, the requirement for a high voltage supply is disadvantageous in some applications, and some types of memory devices, for example serial memory devices, do not have address pins for serving a dual function. Moreover, mask and fusible link ROM is not useful for storing device information for memory devices requiring device information that cannot be obtained until the device is completed.